The present invention is particularly concerned with axial flow rotary valve arrangements that have long windows (i.e. window lengths, as measured in the axial direction, greater than 50% of the cylinder bore diameter), in order to maximise the breathing capacity of the internal combustion engine to which the rotary valve assembly is fitted. Maximum breathing capacity is a dominant consideration in modern engines, where manufacturers seek to gain the greatest power output from the smallest engine size for fuel consumption and emissions reasons.
During rotation of an axial flow rotary valve, openings in the periphery of the valve are arranged to periodically communicate with a similar window in the cylinder head that opens directly into the combustion chamber. Alignment between the opening and the window allows the passage of gas from the valve to the combustion chamber or vice versa. During the compression and power strokes the periphery of the valve blocks the window in the combustion chamber. The valve is typically supported by bearings located either side of a centrally located cylindrical portion of the valve in which the opening (or openings) in the valve's periphery is located. The valve and its bearings are housed in a bore in the cylinder head in such a fashion as to ensure the cylindrical portion can rotate whilst always maintaining a small radial clearance to the bore.
Large numbers of rotary valve arrangements have been proposed but none have achieved commercial success. One of the major contributing factors to this lack of success is the failure to design a satisfactory gas and oil sealing arrangement.
U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) disclose rotary valve arrangements which rotate with a small predetermined clearance to the cylinder head bore in which the rotary valve is housed and gas sealing arrangements using arrays of floating seals. A system of four or more separate sealing elements forms a floating seal grid around the window. The sealing elements are loaded against the periphery of the cylindrical portion of the valve. The cylindrical portion typically extends a small distance past the axial extremities of the array of floating seals.
The function of such an array of floating seals is to trap the high pressure gases within the rectangle formed by the outer surfaces of these seals. The effectiveness of this sealing system depends on its ability to seal the zone at the point of intersection of the individual sealing elements. As the abutting seals must be free to move independently of each other (to accommodate thermal expansion and manufacturing tolerances), there will always be a small gap at each intersection point. As there are at least four such intersection points per assembly, the total leakage gap has the potential to be large. U.S. Pat. No. 5,526,780 (Wallis) introduced the concept of “total effective leakage area” (TELA) as a means of quantifying the leakage area of a particular sealing arrangement.
Both the sealing systems disclosed in U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) do not work satisfactorily due to excessive leakage from the seal pack. So great is this leakage that it is unlikely that the engines using these arrangements will be able to be started using conventional starter motors. Leakage from the operating engine will be so great that the efficiency will be unacceptably low and the exhaust emissions unacceptably high.
The arrangements shown in both U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) seal the high pressure gases in a similar manner and the cause of this excessive leakage is the same for both arrangements. During compression and combustion the high pressure gases in the cylinder push the circumferential seals away from the end of the axial seals leaving an “L” shaped clearance cavity through which the high pressure gases can escape. The side of the “L” shaped clearance is formed between the axially innermost surface of the circumferential seal (end seal in the terminology of U.S. Pat. No. 4,036,184) and the axially innermost surface of its slot (notch in the terminology of U.S. Pat. No. 4,036,184). The bottom of the “L” shaped clearance is formed between the bottom of the circumferential seal and the bottom of its slot.
High pressure combustion gases fill the “L” shaped clearance volume and travel transverse to the axial direction towards both ends of the circumferential seal. Gas trapped in the portion of the “L” shaped clearance volume located circumferentially outside the axial seals (side seals in the terminology of U.S. Pat. No. 4,036,184) can discharge into the clearance, which is at or near atmospheric pressure, between the periphery of the valve and the bore in which the valve is housed. In both U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) this gas is discharged from between the axially innermost face of the circumferential seal and the axially innermost face of the slot in which the circumferential seal is housed. In the case of U.S. Pat. No. 4,036,184 (Guenther), the gas is additionally discharged between the ends of the circumferential seal and the adjacent end walls of the slot. As there are four corners from which this discharge can take place, the total leakage area is very large. In the absence of specific measures, of which none are disclosed, to control the discharge from the portion of the circumferential seals located outside the axial seals, leakage will be unacceptably high for any modern engine.
These sealing problems were identified and addressed in U.S. Pat. No. 5,526,780 (Wallis). It disclosed a sealing arrangement where the TELA was in the order of one thirtieth ( 1/30) that of the arrangement found in U.S. Pat. No. 4,852,532 (Bishop). The typical TELA of the seal arrangement in U.S. Pat. No. 5,526,780 (Wallis) was 0.02 mm2 which is less than the leakage area of a conventional piston ring.
Although the arrangement disclosed in U.S. Pat. No. 5,526,780 (Wallis) satisfactorily addressed the gas leakage issues it required two additional sealing elements and was found to have other problems. In particular this arrangement is particularly difficult to assemble and has excessive crevice volume, particularly when measured relative to the combustion chamber volume on engines with small cylinder capacity. The mounting of the ring seals in the periphery of the valve means the valve has to be larger in diameter than would have otherwise been required. In addition the ring seals at either end of the openings in the valve periphery have large sealing areas between the sealing rings and the valve. These sealing areas are subject to full combustion pressure. Consequently friction drive losses are high. Finally, the seal arrangement is very difficult to assemble as the inner partial ring seals have to be aligned during assembly such that the inner ends of these inner partial ring seals sit outside the small lugs at either end of the axial seals. As the required clearance between the lugs and the inner end of these inner partial ring seals is small, correct alignment during assembly is very difficult and not conducive to high volume production.
Although the crevice volume issues relating to the arrangement disclosed in U.S. Pat. No. 5,526,780 (Wallis) were extensively considered, it was subsequently found that in engines with small cylinder capacity the crevice volume was still sufficiently large to adversely affect the engines performance. The fuel/air mixture in these crevice volumes cannot be burned during the normal combustion process and this consequently results in poor engine fuel economy and performance, and high exhaust emissions. Crevice volumes remote from the spark plug are particularly detrimental, as the expanding flame front pushes unburnt gases into these crevice volumes and the rapidly increasing cylinder pressure means the density of the unburnt gases in the crevice volume rises rapidly. Consequently the mass fraction of the unburnt gases trapped in the crevice volume is much greater than the volume fraction of the crevices.
A feature of sealing arrangements of the type disclosed in the present invention and in U.S. Pat. No. 5,526,780 (Wallis) is the use of the high pressure cylinder gas to actuate the sealing elements. Hence, the greater the pressure required to be sealed, the greater is the closing force applied between the seal and the valve and between the seal and its sealing face in its respective slot. This can only be achieved by allowing the high pressure gas to migrate into those areas surrounding the sealing elements in their slots. The volume occupied by this gas is crevice volume since the air/fuel mixture cannot be burned in these areas during the normal combustion process.
The excessive crevice volume in U.S. Pat. No. 5,526,780 (Wallis) is the result of two issues. Firstly, the outer ring seals extend around the entire periphery of the valve and the inner ring seals extend around approximately 75% of the periphery of the valve. As a result, there is a large crevice volume formed between these seals and the mating groove in the valve and between the ring seals themselves. This problem was exacerbated by the fact that this crevice volume was located a large distance from the spark plug, and consequently the density of the mixture filling this area was high and consisted mainly of unburnt gases. Secondly, the ring seals were located at the end of the axial seals leaving a long cavity between the axial extremity of the window and the ring seals. These two issues resulted in unacceptably large crevice volumes with resulting poor performance and high emissions.
Thus in the prior art there are at least three gas sealing arrangements proposed to seal a rotary valve of the type that operates with clearance between the rotary valve and the bore in which it is housed. Two of these solutions U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) do not seal adequately. The third solution U.S. Pat. No. 5,526,780 (Wallis) addresses the sealing issue but introduces other problems.
A successful gas sealing system should preferably satisfy six criteria. Firstly, it should seal high pressure combustion gas with a minimum of leakage. This leakage is referred to as “blow-by”. Blow-by contains unburned hydrocarbons that are tightly regulated by emissions legislation around the world. Secondly, a successful gas sealing system should have minimal crevice volume. Thirdly, the arrangement must be capable of preventing the blow-by gases being discharged into the exhaust port where they appear as HC (hydro carbon) exhaust emissions. Fourthly, the gas sealing elements should produce minimal drag on the rotary valve in order to minimise frictional losses of the engine. Fifthly, the assembly should be capable of easy assembly in a mass production environment. Finally, the assembly should be capable of economic manufacture in a mass production environment. None of the prior arrangements provide solutions to all these criteria.
The present invention utilizes an array of axial seals and circumferential seals surrounding a window in the cylinder head. The closest prior art to this arrangement is found in U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop). Both suffer from excessive leakage from the seal pack as previously described. U.S. Pat. No. 4,036,184 (Guenther) relates to a stratified charge radial flow rotary valve engine and describes an array of four sealing elements surrounding a window. U.S. Pat. No. 4,852,532 (Bishop) relates to an axial flow rotary valve engine and describes an array of four sealing elements surrounding a window.
Radial flow rotary valves of the type described in U.S. Pat. No. 4,036,184 (Guenther) with a single rotary valve per cylinder require the inlet and exhaust opening in the valve's periphery to be axially offset to one another. This clearly limits the axial opening length of both the inlet and exhaust peripheral opening. This, combined with the fact that the valve rotates at one quarter (¼) of the engine speed, means the arrangement will necessarily have very limited breathing capacity.
The present invention is particularly directed at axial flow rotary valves which rotate at one half (½) of the engine speed. In these arrangements the openings in the valve periphery overlap axially and are offset circumferentially. Consequently the openings may be very long (typically greater than 80% of the cylinder bore diameter). These larger openings, combined with the fact that the valve rotates at half engine speed, means the breathing capacity of such an arrangement is far in excess than anything that may be obtained with a single radial flow valve per cylinder.
The ability to form long openings in the valve periphery introduces design constraints on axial flow rotary valves that are not present on arrangements employing a single radial flow rotary valve per cylinder. Unlike radial flow valves of the type described in U.S. Pat. No. 4,036,184 (Guenther), the only place an axial flow arrangement can support the axial seals is outboard of the axial extremities of the openings in the valve periphery. In arrangements employing a single radial flow valve per cylinder there is minimal requirement for axial seal support outside the axial extremities of the opening in the valve periphery, as there are “bridges” of complete valve diameter between adjacent openings to support the axial seal. Consequently in radial flow rotary valves of the type disclosed U.S. Pat. No. 4,036,184 (Guenther), the circumferential seals may be placed close to the adjacent window without adversely affecting the crevice volume.
In axial flow rotary valve arrangements of the type shown in U.S. Pat. No. 5,526,780 (Wallis) and U.S. Pat. No. 4,852,532 (Bishop) the axial seals spanning a single long opening must extend some distance axially past the end of the window, in order that they have sufficient bearing area on the periphery of the valve. Placing the circumferential seal axially outboard of the axial seal results in a large crevice volume between the axial extremities of the window and the circumferential seal. As this crevice volume is remote from the spark plugs, it will be filled predominately with unburned gases further exacerbating the problem caused by this crevice volume.
In U.S. Pat. No. 4,036,184 (Guenther) the axial seals are shown as being radially small and hence relatively flexible compared to the circumferential seals that are shown to be radially large and hence relatively stiff. The relative size and stiffness of these elements is presumably linked to the sealing function although there is no explanation in the patent. This is, however, an arrangement that cannot work.
The circumferential seal of the type depicted in U.S. Pat. No. 4,036,184 (Guenther) is unsatisfactory as it is too stiff to conform to the surface of the rotary valve. Thermal and mechanical loads distort the surface of the valve during operation. Even a statically perfectly matched seal will not seal against the valve's periphery during operation unless it is flexible enough to conform to the changing shape of the valve surface. This will inevitably result in leakage across the top of the seal and destabilisation of the sealing mechanism. The presence of high pressure gas between the valve and the mating surface of the circumferential seal will result in the seal being pushed away from the valve surface rather than being pushed into contact with the valve surface. The result will be massive leakage across the sealing surface of the circumferential seal.
The flexible axial seals are pressed against the periphery of the valve by means of a continuous wave spring. This spring arrangement would act to deflect the axial seals into the opening in the valve's periphery, hence potentially causing them to have a collision with the closing edge of the opening and destroying the seals. Axial seals must therefore have adequate stiffness and the spring arrangement appropriately designed to ensure this does not occur. In this prior art arrangement it is paradoxical that the circumferential seals (that must conform to the peripheral surface of the valve in order to seal) are radially deeper in section compared to the axial seals, which are required to be stiff.
In the case of an arrangement employing a single radial flow rotary valve per cylinder as described in U.S. Pat. No. 4,036,184 (Guenther) with three separate openings spaced axially along the rotary valve, each separated axially from one another, there is clearly less requirement for axial seal stiffness. However this arrangement would have very little breathing capability. U.S. Pat. No. 4,036,184 (Guenther) states that a twin valve arrangement is preferable because it allows central location of the pre-combustion chamber. A radial flow arrangement with two rotary valves per cylinder (ie. separate valves for inlet and exhaust) would in part address this breathing issue by allowing the use of long openings in the valve but would not work with the axial seal arrangement as depicted in U.S. Pat. No. 4,036,184 (Guenther). It is assumed that the twin valve arrangement would (although it is not shown) maximise the available opening (and window) length to improve breathing capability. This arrangement could only be made to work by substantially increasing the stiffness of the axial seals by increasing their depth. A twin valve arrangement would create additional crevice volume and leakage problems as there are now two seal arrays, which doubles both the TELA and crevice volume. In the absence of deep axial seals the wave spring will merely push the axial seal into the opening in the valve's periphery with resulting impact against the closing edge of the opening. A similar situation exits in axial flow rotary valve arrangements of the type shown in U.S. Pat. No. 5,526,780 (Wallis) and U.S. Pat. No. 4,852,532 (Bishop).
In any workable arrangement using long windows the radial depth and the stiffness of the axial sealing elements must be considerably greater than the radial depth and stiffness of the circumferential seals. Such an arrangement is disclosed in U.S. Pat. No. 4,852,532 (Bishop).
In addition to excessive stiffness problems the circumferential seals depicted in U.S. Pat. No. 4,036,184 (Guenther) have an additional problem. The large size of the seal means there will be an excessive crevice volume around the circumferential seal. In the case of U.S. Pat. No. 4,852,532 (Bishop), the long length of the circumferential seal results in excessive crevice volume around the seal despite the fact it is radially small.
Neither U.S. Pat. No. 4,036,184 (Guenther) nor U.S. Pat. No. 4,852,532 (Bishop) has a satisfactorily means of preventing the leakage from the seal pack entering the exhaust system and becoming an emissions problem. Today, the exhaust emissions from most engines are tightly regulated. In both U.S. Pat. No. 4,036,184 (Guenther) and U.S. Pat. No. 4,852,532 (Bishop) leakage past the sealing element will be delivered to the inlet and exhaust ports. Leakage circumferentially outboard of the trailing axial seal will end up in the inlet port (see FIG. 7 of U.S. Pat. No. 4,853,532) where it will be recycled harmlessly back into the engine. Leakage circumferentially outboard of the leading axial seal will end up in the exhaust port where it will be discharged from the exhaust as unburnt hydrocarbons. As leakage occurs from both ends of the circumferential seal outside the circumferential extremities of the axial seals approximately half of the total leakage will end up in the exhaust port and half in the inlet port. Such an engine will be unacceptable from an emissions perspective.
In the case of U.S. Pat. No. 4,036,184 (Guenther) the circumferential seals must be housed in blind ended slots (notches in the terminology of U.S. Pat. No. 4,036,184) in the cylinder head. There is no known mass production method of forming these blind ended slots. They could be manufactured by electro discharge machining but this is a slow process and the high depth of these slots would make the process even slower. In the case of U.S. Pat. No. 4,852,532 (Bishop) the circumferential seals are housed in circumferential slots that extend around the entire housing and are therefore easy to manufacture. However, this feature is the cause of the crevice volume problems.
Finally these prior art arrangements are difficult to assemble in a mass production situation. Each of the individual sealing elements has to be individually held in their retracted position whilst the rotary valve is assembled or the head assembly (including the fitted seals) is transported. In a multicylinder head assembly every seal will have to be fitted and retained individually before this subassembly is sent to have the valve assembled into the head.
In addition to requiring a gas sealing system, axial flow rotary valves usually also require an oil sealing system. The bearings supporting the rotary valve are typically lubricated with oil. In most instances the valve is also cooled with oil that is pumped through the valve. A successful oil sealing system must satisfy two criteria. Firstly, it must prevent the axial inward leakage of this oil into the central cylindrical portion of the valve and prevent the axial outward movement of blow-by gases into the oil system. Secondly, it must act in combination with the gas sealing elements to manage the passage of blow-by gases into an area where it can be disposed of without creating emissions. U.S. Pat. No. 4,036,184 (Guenther) is silent on this aspect.
U.S. Pat. No. 4,852,532 (Bishop) uses the circumferential seal as both a gas sealing and an oil sealing element. Such an arrangement has been demonstrated to not work as a satisfactory oil seal. During the induction stroke negative pressure in the cylinder pulls the circumferential sealing element axially inward against the axially inner wall of its seal slot. This opens up a gap between the axially outer face of the circumferential seal and the axially outermost wall of its seal slot. Oil driven by the oil pressure and the negative cylinder pressure can now enter this gap and deposit oil into the volume under the circumferential seal. During the compression stroke the circumferential seal is pushed axially outward against the axially outer wall of its seal slot thus opening up a gap between the axially inner face of the circumferential seal and the axially inner wall of the seal slot. High pressure gas from the cylinder enters the cavity under the circumferential seal and blows this oil out with the leakage into the inlet and exhaust ports.
U.S. Pat. No. 5,509,386 (Wallis et al) discloses a gas and oil sealing arrangement using an array of floating seals to affect the gas sealing and face seals to affect the oil sealing. The floating array of seals consists of two axial seals and four ring seals. The axial seals are located in slots in the cylinder head bore and the ring seals are located in grooves in the valve. Oil sealing is affected by non-rotating annular members located axially outboard of the ring seals. In this arrangement, blow-by gases leaking past the ring seal are trapped in the annular cavity formed radially between the outer diameter of the valve and the cylinder head bore, and axially between the ring seal and the non-rotating annular member. The non-rotating annular member is designed to ‘blow-off’ as a result of pressure build up in the annular cavity, and discharge the blow-by gases into the oil system. In practice, these blow-by gases contain unburnt fuel which when discharged into the oil system over time, heavily contaminates the oil degrading its lubricating properties. The continuous discharge of unburnt fuel into the lubricating oil system causes the volume of oil in the system to increase over time.
Although methods of ameliorating this problem are disclosed in U.S. Pat. No. 5,509,386 (Wallis), none are totally effective as they merely reduce the frequency that the non-rotating annular member is blown off the radially disposed face. The only effective solution is to eliminate the discharge of blow-by across the non-rotating annular member. One solution is disclosed that may overcome this problem but has the additional complication of two pressure relief valves per rotary valve assembly and additional plumbing.
The present invention seeks to provide a sealing system for a rotary valve assembly that ameliorates at least some of the problems of the prior art.